When green asparagus is planted, the asparagus spears do not grow at a constant rate. Thus a row of asparagus will contain spears of various lengths, only the tallest of which will be suitable for picking.
Asparagus harvesters have been designed for picking asparagus spears, but those harvesters (a) are slow and (b) pick spears of all heights. Consequently, such harvesters are wasteful. Indeed, they are seldom used, for it is more economical to use manual labour to pick asparagus spears every two days over the four months period when asparagus plants produce spears of acceptable length. Manual picking of asparagus spears, however, is uncomfortable and thus unattractive work, and labour for this task is often difficult to find at the harvest period.